Internal combustion engines commonly use an engine coolant to cool and maintain the operating temperature of the engine. As the coolant is routed through the engine, the coolant absorbs thermal energy from the engine causing the coolant to expand due to thermal expansion. The expanded coolant takes up more space in the engine and any excess volume (overflow) of engine coolant from the engine moves to an overflow reservoir bottle. Subsequently, the space for the air (air volume) inside the overflow reservoir bottle reduces and any air inside the bottle compresses. The compressed air inside the overflow reservoir bottle can be at a higher pressure than the air outside of the overflow reservoir bottle. This pressure differential causes the air to leave the bottle to bottle to equalize the air pressure between the inside and the outside of the bottle. The air leaves the overflow reservoir bottle usually via a neck/cap interface. In addition to air leaving the bottle, the coolant itself may slowly seep through the neck/cap interface as the coolant splashes and sloshes inside the overflow reservoir bottle, for example, as the vehicle traverses a bumpy road and the coolant inside the bottle is agitated and splashes near and possibly through the neck/cap interface. Any seepage may result in coolant getting collected on the outside of the overflow reservoir bottle, especially at a seam of the overflow reservoir bottle, giving the appearance of a recovery bottle or cooling system issue.
Therefore, the appearance problem can be avoided by pursuing an overflow reservoir bottle cap that would allow air to vent out of the overflow reservoir bottle while eliminating or reducing the engine coolant from splashing through the cap.